Today Kerry Kennedy is out to convince the jury that she took the sleeping pill by accident. And had no idea it was putting her asleep behind the wheel to 54 year old niece of president John F. Kennedy is on trial for drugs driving. A misdemeanor charge for the danger she posed on the road. I remember. Getting on the highway and then I have no memory. Until I was done. In July 2012 Kennedy side swiped a tractor trailer while in -- white Lexus on the way to the gym no one was hurt. But when police stopped her Kennedy failed field sobriety tests there's no question she was a danger. The question is should she be held legally responsible for that -- misdemeanor trial defense attorneys say Kennedy didn't know she was being put to sleep by appeal mistakenly taking -- sleeping -- with her breakfast cappuccino instead of her thyroid prescription. Her famous family including mother 85 year old Ethel had been standing by her story. -- sister -- testified Tuesday -- Kerry has a reputation for sobriety and general healthy living. But a forensic toxicologist testifying for the prosecution. Said Ambien is fast acting in could have been felt in as little as fifteen minutes. Prosecutors -- Kennedy should have made the decision to pull over or go home. The Kennedys no strangers to public legal battles hoping the power of family. And they're famous name will prevail. From the stand Kennedy denied right off the -- knowingly or willingly taking Ambien before driving if convicted she faces the loss of her driving privileges. And up to a year in jail. Devin Dwyer ABC news New York.

This transcript has been automatically generated and may not be 100% accurate.

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